This invention relates generally to mechanical screw jacks, and more specifically to a method and apparatus for limiting the travel or stroke of a screw in a mechanical screw jack.
Mechanical screw jacks are used in applications in which large loads must be raised and lowered in a reliable, efficient, simple and safe fashion. For example, mechanical screw jacks are often used to raise and lower vehicles and/or towed attachments, including items such as cars, trucks, tractors, trailers, and agricultural implements.
Screw jacks usually include a housing having telescoping housing sections that position the jack linearly between a raised or extended position (i.e., maximum attainable length) and a lowered or retracted position (i.e., minimum attainable length). Screw jacks (by name) typically include a screw jack nut and a screw. The screw is typically in threadable engagement with the nut. A crank or other mechanism can be used to rotate gearing mechanisms (e.g., bevel gears, spur gears, etc.) which cause the screw to move with respect to the nut. Put another way, the gearing mechanism, if used, in conjunction with the screw transforms the rotational motion of the handle into linear motion of the jack. “Screw travel” refers to the movement of the screw with respect to the nut and results in a change in the overall length or height of the jack.
One of the problems associated with mechanical screw jacks is a condition known as “over travel” where the screw travels with respect to the nut beyond a desired, or designed, amount. Over travel can result in damage to the threaded portions of the screw and/or nut or even failure of the entire jack assembly. When there is permanent damage to the threaded portions of the screw and/or nut, a user may experience a phenomena known as a “residual torque effect” independent of the load being borne by the jack. The residual torque can become so great that the jack could seize or lock-up and no longer be useable under any loading circumstances. In addition, over travel can result in failure to stop screw travel possibly at a low torque, with the result that the screw is able to become disengaged from the nut. In this case, the jack could separate or fall apart.
Therefore, it would be desirable to be able to limit the screw travel and be able to apply sufficient sensible excess torque (e.g., torque or cranking effort in substantially excess of that required to typically operate the screw jack, substantial enough that the operator senses the intended travel limit) to the screw with respect to the nut without damaging the threaded portion of the nut, and as such, reduce or eliminate any residual torque effect. It would also be desirable to increase mechanical screw jack life and reduce the likelihood of separation of such mechanical jacks into their component parts during repeated and vigorous use.